Biologists have long sought to identify and explain patterns in the diverse array of marine life histories. The most famous speculation about such patterns is Gunnar Thorson's suggestion that species producing planktonic larvae are rarer at higher latitudes (Thorson's rule). Although some elements of Thorson's rule have proven incorrect, other elements remain untested. With a wealth of new life-history data, statistical approaches, and remote-sensing technology, new insights into marine reproduction can be generated. We gathered life-history data for more than 1,000 marine invertebrates and examined patterns in the prevalence of different life histories. Systematic patterns in marine life histories exist at a range of scales, some of which support Thorson, whereas others suggest previously unrecognized relationships between the marine environment and the life histories of marine invertebrates. Overall, marine life histories covary strongly with temperature and local ocean productivity, and different regions should be managed accordingly.

BackgroundAmong metazoans, retention of functional diet-derived chloroplasts (kleptoplasty) is known only from the sea slug taxon Sacoglossa (Gastropoda: Opisthobranchia). Intracellular maintenance of plastids in the slug's digestive epithelium has long attracted interest given its implications for understanding the evolution of endosymbiosis. However, photosynthetic ability varies widely among sacoglossans; some species have no plastid retention while others survive for months solely on photosynthesis. We present a molecular phylogenetic hypothesis for the Sacoglossa and a survey of kleptoplasty from representatives of all major clades. We sought to quantify variation in photosynthetic ability among lineages, identify phylogenetic origins of plastid retention, and assess whether kleptoplasty was a key character in the radiation of the Sacoglossa.ResultsThree levels of photosynthetic activity were detected: (1) no functional retention; (2) short-term retention lasting about one week; and (3) long-term retention for over a month. Phylogenetic analysis of one nuclear and two mitochondrial loci revealed reciprocal monophyly of the shelled Oxynoacea and shell-less Plakobranchacea, the latter comprising a monophyletic Plakobranchoidea and paraphyletic Limapontioidea. Only species in the Plakobranchoidea expressed short- or long-term kleptoplasty, most belonging to a speciose clade of slugs bearing parapodia (lateral flaps covering the dorsum). Bayesian ancestral character state reconstructions indicated that functional short-term retention arose once in the last common ancestor of Plakobranchoidea, and independently evolved into long-term retention in four derived species.ConclusionWe propose a sequential progression from short- to long-term kleptoplasty, with different adaptations involved in each step. Short-term kleptoplasty likely arose as a deficiency in plastid digestion, yielding additional energy via the release of fixed carbon. Functional short-term retention was an apomorphy of the Plakobranchoidea, but the subsequent evolution of parapodia enabled slugs to protect kleptoplasts against high irradiance and further prolong plastid survival. We conclude that functional short-term retention was necessary but not sufficient for an adaptive radiation in the Plakobranchoidea, especially in the genus Elysia which comprises a third of all sacoglossan species. The adaptations necessary for long-term chloroplast survival arose independently in species feeding on different algal hosts, providing a valuable study system for examining the parallel evolution of this unique trophic strategy.

Chemical communication between sperm and egg is a critical factor mediating sexual reproduction. Sperm attractants may be significant evolutionarily for maintaining species barriers, and important ecologically for increasing gamete encounters. Still unresolved, however, are the functional consequences of these dissolved signal molecules. Here, we provide experimental evidence that sperm chemoattraction directly affects the magnitude of fertilization success. The recent discovery of L-tryptophan as a potent attractant to red abalone (Haliotis rufescens) sperm affords the opportunity to quantify how navigation affects gamete interactions. Sperm behavioral responses to manipulations of the natural tryptophan gradient around individual eggs reveals that both chemotaxis and chemokinesis significantly promote contacts. Our results show further that attractant release by means of diffusion effectively doubles the target size of red abalone eggs, which in turn significantly increases fertilization success. Although long theorized as potential barriers to hybridization, species-specific sperm attractants in red and green (Haliotis fulgens) abalone are only minor contributors to maintaining reproductive isolation. Because abalone typically live in dense, multispecies aggregations, chemically mediated navigation would prevent sperm from pointlessly tracking heterospecific eggs. Thus, even though reproductive isolation fundamentally resides at the level of membrane recognition proteins, species-specific sperm attractants may evolve to locate the right target within mixed gamete suspensions of closely related species.

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